How to size tires to fit on your 1965 to 1973 Mustang.
Putting bigger tires on your car can not only make your car look better, it can make your car handle better as well. Trying to decide just how big a tire you can install on your car can be somewhat intimidating. Too small, and you won’t be happy. Too big and you will ruin tires and perhaps worse cause your car to crash and ruin your ride. Not to mention the noise they will make when you hit bumps in the road and when they hit your fenders or your inner wheel well housings.
How big a tire you can install depends on the back spacing of the wheel that you are trying to use. Backspacing is the distance between the back face of the wheel that is bolted against the brake drum or rotor by the lug nuts, and the back side of the rim of the wheel.
To measure a wheels back spacing, lay the wheel on the ground with the fancy outside part down. Lay a yardstick or other flat item across the top of the wheel. That would be the part that faces the leaf spring. Next measure the distance from the lower part of the yardstick to the flat part of the wheel that bolt up to the brake assembly on your car. That is the backspacing of that wheel.
When comparing the two wheels of the same width, the greater the backspacing, the further the wheel and tire set into the fender. If you use too much backspacing, the tire will rub on the inside of the inner fender well either in the front or the rear. If you use too little backspacing, the tire will hit on your fender and tear up the tire and your fender.
A good tire for a 1965 or 1966 mustang is a 17″ x 8″ wheel with a 4 3/4″ back space. A good tire to use with this is a 235/45-17″ tire. This wheel and tire combination does not require rolling fenders or cause wheel rub. You can also use this combination on all four tires and use the same size tire all around your car.
One problem on 1965 to 1973 Mustangs that I have read about is the front end upper ball joint clearance. A 17″ x 8″ wheel will fit these cars much better than a 16″ x 8″ wheel? because the upper ball joint is almost right in line with the edge of the rim on a 16″ wheel which limits the back space. But on a 17″ wheel there is 1/2 inch more backspace because the rim is higher allowing the rim to pass over the top of the upper control arm. This gives more clearance to the tire and fender. 18″ and larger diameter wheels are better, if you can find them with the correct back space and bolt pattern.
1967 to 1973 Mustangs have larger fender openings so tire rubbing is a less of a problem. Too much back spacing allows tires to rub on the frame when turning, especially to park or make sharp maneuvers. Be sure to check the tire to frame clearance if you use a wheel with a lot of back spacing.
Use this chart as a guide to select tire rim combinations but remember other factors come into play as well. Tire shape and the condition of your Mustang’s suspension also affect tire clearance. A worn out stock suspension with excessive bounce will be a more likely to rub than a new firm suspension.